1. Field of the Invention
The present invention relates to interior permanent magnet (IPM) rotors and methods and apparatuses for manufacturing the same.
2. Description of the Related Art
Interior permanent magnet (IPM) motors having permanent magnets embedded in a rotor are known in the art. A technique of using resin magnets containing well-known ferromagnetic powder and a resin binder is known as interior permanent magnet (IPM) rotors that are used for such IPM motors. For example, in Japanese Patent Application Publication No. 2002-44915 (JP 2002-44915 A), a molten resin magnet material is introduced into slits of a cylindrical rotor placed in a mold while being magnetically oriented by permanent magnets in the mold.
Each slit has an arc shape in section and has a pair of ends and a bottom portion. The ends are located close to the outer periphery of a rotor core. The bottom portion is located inward of the ends in the radial direction of the rotor core. Gates provided in the mold in order to inject the resin magnet material therethrough are located so as to face the bottom portion in the axial direction of the rotor core.
The resin magnet material that is introduced into the slits has a lower orientation rate and a lower magnetization rate in the bottom portion than in the ends. Each resin magnet formed as the resin magnet material injected through the gate becomes solidified in the slit includes a high magnetization portion and a low magnetization portion. The high magnetization portion is formed as the resin magnet material moves from the bottom portion to the ends and thus becomes solidified at a high orientation rate and a high magnetization rate. The low magnetization portion is formed as the resin magnet material stays around the bottom portion and thus becomes solidified at a low orientation rate and a low magnetization rate.
Accordingly, the overall magnetization rate of the resin magnet is reduced, and the overall magnetic flux density of the rotor is reduced. This reduces the effective amount of magnetic flux crossing a stator coil of the motor, which causes reduction in output torque of the rotor.